Direct thermal imaging is a technique in which a substrate bearing at least one image-forming layer, which is typically initially colorless, is heated by contact with a thermal printing head to form an image. In direct thermal imaging there is no need for ink, toner, or thermal transfer ribbon. Rather, the chemistry required to form an image is present in the imaging member itself. Direct thermal imaging is commonly used to make black and white images, and is often employed for the printing of, for example, labels and store receipts. There have been described in the prior art numerous attempts to achieve multicolor direct thermal printing. A discussion of various direct thermal color imaging methods is provided in U.S. Pat. No. 6,801,233 B2.
In the method of the present invention, a direct thermal imaging member having more than one image-forming layer is addressed by a thermal printing head to provide a colored image. The imaging member is addressed in more than one pass of a thermal printing head, at least one pass being at a different speed from at least another pass. Optionally, the imaging member is preheated to a different extent in at least one pass than in at least another pass.
In U.S. Pat. No. 6,801,233 B2 there is described and claimed a direct thermal imaging system in which one or more thermal printing heads can form two colors in a single pass on the imaging member. The printer can form these multiple colors by addressing two or more image-forming layers of the imaging member at least partially independently from the same surface so that each color can be formed alone or in selectable proportion with the other color(s). In a preferred embodiment, a printer can form three colors on three image-forming layers which may be carried by the same surface of a substrate.
Thermal printing devices with variable printing speed are known in the art, as described, for example, in U.S. Pat. Nos. 5,319,392 and 6,078,343. These can be direct thermal or thermal transfer printers. In general, the speed of thermal printers depends upon the nature of the image to be printed. Thus, low-quality direct thermal images (such as store receipts) may be printed at speeds of 3 inches/second or more. Thermal transfer printing of photographic quality is typically carried out at speeds of less than 1 inch/second.
Preheating of a thermally activated printing head is described, for example, in U.S. Pat. No. 5,191,357 which describes a recording apparatus for performing recording on a recording medium where the apparatus includes a plurality of recording elements and a control unit for selectively providing energy having a level lower than an actual recording level. It is also known to preheat a thermal transfer ink layer in a thermal transfer imaging method. For example, U.S. Pat. No. 5,529,408 discloses a thermal transfer recording method wherein the thermal transfer ink layer is preheated prior to having energy applied thereto in order to initiate transfer of the ink to a receiving material.
As the state of the thermal imaging art advances, efforts continue to be made to provide thermal imaging materials and thermal imaging methods that can meet new performance requirements.